The invention relates firstly to a method for removing a structure or a part of a structure which is erected on an underwater bed or anchored to the underwater bed. The invention also relates to a method for positioning a structure or a part of the structure on an underwater bed or anchoring it to the underwater bed.
In practice, structures positioned in the sea, such as offshore platforms, are constructed from a substructure which is erected on the seabed or floats on the surface, in which case it is anchored to the seabed, including, for example, substructures of so-called xe2x80x9cSparxe2x80x9d and xe2x80x9ctension-legxe2x80x9d platforms, and a superstructure which comprises a deck provided with deck modules. When a structure of this type is being positioned, the substructure is positioned first, after which the unitary superstructure is arranged thereon. In a known method the substructure is positioned by tilting the substructure, which lies approximately horizontally on a transport and hoisting vessel, into the water via a tilting device and then lowering it in an approximately vertical position. In a known method for removal of a structure using the same vessel, the superstructure is removed first, after which the substructure is completely or partially removed. The substructure is hoisted upwards using a hoisting mechanism arranged on the transport vessel itself and is then placed horizontally on the vessel via the tilting device.
The drawback of a method of this type is that the hoisting means for lowering or raising the substructure are arranged on the transport vessel itself. This requires additional structural measures on the vessel, leading to a more expensive vessel.
An alternative solution for positioning the substructure is shown in WO 90/03470. This publication does not disclose a hoisting mechanism, but rather a submersible barge which is tiltably connected to a transport vessel. The submersible barge is provided with ballast tanks which can be filled with air or water in order to raise or lower the substructure when connected to the barge. This solution also has the drawback that a complicated structure is required on the transport vessel.
The object of the invention is to provide a method which allows the transport vessel to be of more simple design.
The object of the invention is achieved by hoisting or lowering the structure using a separate hoisting vessel on which the hoisting mechanism is arranged. For example, it is possible for an existing crane ship to be used. This has the advantage that the transport vessel does not have to have specially developed hoisting means, and consequently costs are saved. An additional advantage is that the hoisting means may also be used for other purposes.
The transport vessel for use in this method is preferably designed with attachment means which comprise at least one tiltable arm which is provided with suspension means which can act on the structure.
In a preferred embodiment the suspension means comprise a suspension hook. The suspension hook is situated above the surface of the water. For example when being removed, the structure is hoisted upwards by the hoisting vessel, after which the suspension hook can engage thereon. The advantage of this is that the suspension hook engages on a part of the structure which is situated above water and can therefore easily be checked for strength beforehand.
In another embodiment the suspension means comprise a support foot which can act on a lower part of the structure. The advantage of this support foot is that it can act on a lower and more sturdy section of the structure, so that the risk of the structure collapsing when it is being raised or lowered is reduced.
In a third embodiment a second tiltable arm is connected to one end of the at least one tiltable arm, resulting in a tilting mechanism with two tilting points, which has the advantage that the structure is well supported through the entire tilting movement. Furthermore, in this embodiment of the attachment means the suspension means can comprise cables which are arranged on the transport vessel on the side of the tilting arms and hang downwards from the vessel. The lower ends of the cables are provided with attachment members for acting on a part of the structure which is to be positioned or removed. The advantage of this is that the length of the cables can be easily adapted, so that the location of engagement on the structure is easy to vary and any desired part of the structure can be supported and taken away.